1. Field of the Invention
The present invention relates to an image forming apparatus as exemplified by a printer, a copier, and a facsimile machine and particularly to a cleaning technique for a fixing device upon a recovery from a sheet jam.
2. Description of the Related Art
Conventionally, a jam has frequently occurred in a fixing portion in an electrophotographic image forming apparatus. Upon an occurrence of the jam in the fixing portion, there is a strong possibility of adherence of unfixed toner, which is not fixed to a sheet yet, to a heating roller and a pressure roller. For example, in the case of a jam of a sheet that has not yet passed the fixing portion after passing a registration roller, the sheet may be accordion-folded before fixing rollers. This accordion-folded sheet is pressed against the fixing rollers to smear the fixing rollers. Further, in the case of a fixing-portion residence time overrun jam where a sheet remains in the fixing portion for longer than a predetermined period, the apparatus stops with the sheet stuck in the fixing portion. Therefore, there is a likelihood that the unfixed toner on the sheet is adhered to the fixing rollers.
Most of cases where the heating roller and the pressure roller constituting the fixing rollers are smeared are caused by jams occurring in the vicinity of the fixing portion. In the case of the smear caused by a jam, there is a problem of smearing several sheets to be printed immediately after the jam is removed, thereby reducing print quality.
On the other hand, Japanese Unexamined Patent Publication No. 2003-149979 discloses a cleaning method according to which residual toner is removed not by applying an oil or the like to fixing rollers to prevent the toner from remaining thereon, but by passing blank sheet(s) to transfer the residual toner from the fixing rollers.
In this publication, there are proposed a method according to which a cleaning operation is manually performed upon being instructed by section of an operation section by a user or a service person, and a method according to which the cleaning operation is automatically performed after the lapse of a predetermined period or after a specified number of sheets are printed. According to either method, the cleaning operation is performed in a standby state where an apparatus main body does not perform printing.
Accordingly, in the case of manually performing the cleaning operation, if the user is not aware of the cleaning operation, no cleaning operation is performed, wherefore there is a problem of smearing several sheets immediately after the jam is removed as with the conventional practice, thereby reducing the print quality. Even in the case of automatically performing the cleaning operation, there is no effect on image smears immediately after an occurrence of a jam since the cleaning operation is performed after the lapse of the predetermined period or after the specified number of sheets are printed. Further, in spite of differing degrees of smearing the rollers depending on a frequency of occurrence of jams during the predetermined period or within a period during which the specified number of sheets are printed, the cleaning operation is performed at constant intervals. This causes a problem that it is continued to perform no cleaning operation despite the gross smear, thereby reducing the print quality, and a problem that the cleaning operation is performed despite only a small degree of smearing, thereby wasting sheets.
Further, since the cleaning operation is performed only in the standby state, if a jam occurs during a job, the cleaning operation is performed after the remaining job is carried out upon a recovery from the jam. Therefore, there is a problem of reducing the print quality in the remaining job.